Which of the following scenarios can be classified as a negative feedback loop? a increase in blood pressure to reduce heart rate
b increase in testosterone to help increase a fight-or-flight response
c increase in oxytocin caused by increased contractions
d increase in platelet production to help increase blood clotting

The scenario that can be considered an example of a negative feedback loop is increase in platelet production to help increase blood clotting (option d).

Explanation:

Negative feedback loop is what occurs when a change occurs in the body, producing a response that decreases or stops that change, with the intention of maintaining homeostasis.

When there is a wound that produces bleeding, a series of mechanisms are activated, the coagulation cascade, which promotes the release of factors that allow the formation of clots. The increased release of platelets implies negative feedback because it helps to stop the change that occurs in the body, the loss of blood, through the formation of clots.

One thing that can cause confusion is the fact that this response also involves positive, though limited, feedback that promotes the production of clotting factors and platelets to increase clot formation. What really happens is that the positive feedback promotes the general feedback, which is the negative feedback: clot formation to stop the bleeding, restoring homeostasis.

The other options are not correct because:

    a. Increase in blood pressure to reduce heart rate is not possible.

    b. In the fight and flight response the hormones involved are cortisol and adrenaline, not testosterone.

    c. Increased oxytocin in response to uterine contractions is positive feedback.

the african cheetah.

this species has a relatively low level of genetic

variation because the population was reduced to a small size approxi-

mately 10,000 to 12,000 years ago.

chapter   outline

24.1

genes in populations and the hardy-

weinberg equation

24.2

factors that change allele and genotype

frequencies in populations

24.3     sources of new genetic variation

population   genetics

24

until   now,   we   have   primarily   focused   our   attention   on   genes  

within   individuals   and   their   related   family   members.   in   this  

chapter   and   chapters   25   and   26   ,   we   turn   to   the   study   of   genes   in  

a   population   or   species.   the   field   of

population   genetics

  is   con-

cerned   with   changes   in   genetic   variation   within   a   group   of   indi-

viduals   over   time.   population   geneticists   want   to   know   the   extent  

of   genetic   variation   within   populations,   why   it   exists,   and   how   it  

changes   over   the   course   of   many   generations.   the   field   of   popu-

lation   genetics   emerged   as   a   branch   of   genetics   in   the   1920s   and  

1930s.   its   mathematical   foundations   were   developed   by   theoreti-

cians   who   extended   the   principles   of   gregor   mendel   and   charles  

darwin   by   deriving   formulas   to   explain   the   occurrence   of   geno-

types   within   populations.   these   foundations   can   be   largely   attrib-

uted to three scientists: sir ronald fisher, sewall wright, and j. b. s.

haldane.   as   we   will   see,   support   for   their   mathematical   theories  

was provided by several researchers who analyzed the genetic com-

position   of   natural   and   experimental   populations.   more   recently,  

population   geneticists   have   used   techniques   to   probe   genetic  

variation   at   the   molecular   level.   in   addition,   staggering   advances  

in   computer   technology   have   aided   population   geneticists   in   the  

analysis   of   their   genetic   theories   and   data.   in   this   chapter,   we   will  

explore   the   genetic   variation   that   occurs   in   populations   and   con-

sider   the   reasons   why   the   genetic   composition   of   populations   may  

change over the course of several generations

air enters through the nose, moves through the nasal cavity, the pharynx, the larynx, enters the trachea, moves through the bronchi andbronchioles till the alveoli.